1. Field of the Invention
The present invention relates to a flow sensor, and more particularly to a thermal micro flow sensor produced by means of semiconductor fabrication technology.
A micro flow sensor has an advantage that it has a very small size and thus it can be installed in a very small space, for example in the inside of a thin gas pipe so that the micro flow sensor detects a gas flow rate. Another advantage of the micro flow sensor is that it consumes very low electric power and therefore it can be employed for use in a flow meter that can operate with a battery power supply and thus it can be installed in an environment where a commercial AC power line is not available. Furthermore, the micro flow sensor can detect flow velocity over a very wide range from a few mm/s to a few ten m/s, and thus only a single micro flow sensor is required to detect flow velocity over such a wide range.
2. Description of the Prior Art
It is known in the art to fabricate a thermal micro flow sensor by forming a fine heating wire on a semiconductor substrate such as a silicon substrate using semiconductor fabrication technology including a fine patterning technique. It is also known to dispose a flow sensor of this type in the middle of a flow path of fluid so as to detect a flow rate on the principle that the heat transfer from the heating wire to the fluid changes depending on the flow rate of the fluid. Flow sensors of this type have advantages that they can be easily mass-produced, they have a small size, they consume very small electric power, and they can easily be connected to other electric circuits. Because of these advantages, flow sensor of this type, particularly in recent years, have become popular as means of detecting a flow rate or flow velocity.
In recent flow sensors of this type, a heating wire is formed in a bridge or cantilever beam shape on a semiconductor substrate so as to improve heat insulation between the heating wire and the semiconductor substrate thereby increasing the sensitivity of sensors and reducing thermal loss.
The heating wire on the semiconductor substrate can be made of various metal materials such as platinum, gold, copper, aluminum, chromium, tungsten, or permalloy (FeNi). An example of a thermal micro flow sensor of the above-described type is disclosed in Japanese Patent Laid-Open No. 3-52028 (1991).
In measuring operation, it is required that the sensor can provide an output signal greater than a required level with low noise. Furthermore, it is desirable that the sensor consume as low electric power as possible. For the above purpose, it is desirable that the current flowing through the heating wire be set to a value in the range from 0.5 mA to 10 mA and the voltage across the heating wire be set to a value in the range from 1 V to 10 V. Furthermore, it is desirable that the heating wire itself have a resistance in the range from 0.5 k.OMEGA. to 10 k.OMEGA. and more preferably the overall resistance be about 1 k.OMEGA..
The power consumption of the thermal flow sensor is equal to the sum of the amount of heat Q.sub.1 transferred from the heating wire to the fluid per unit time and the amount of heat Q.sub.2 removed from the heating wire via the substrate. In the case where good thermal isolation is provided between the heating wire and the substrate, the power consumption is determined by Q.sub.1 wherein Q.sub.1 is proportional to the contact area S between the heating wire and the fluid. It has been concluded experimentally that the contact area S should be less than about 0.02 mm.sup.2 to obtain small power consumption less than a few ten mW.
When the heating wire is made of metal having a resistivity smaller than 10.sup.-7 .OMEGA.m, it is required that the thickness of the heating wire should be less than about 0.1 .mu.m to obtain a heating wire having a width greater than 5 .mu.m and having a resistance of about 1 k.OMEGA. while maintaining the power consumption at a low level less than a few ten mw. However, a bridge or cantilever beam structure having such a thin thickness cannot have enough strength to be formed in a self-supporting fashion on a substrate unless there is provided a supporting structure for supporting the bridge or cantilever beam structure. However, the employment of the supporting structure will result in an increase in the volume of a part that makes no contribution to generation of heat, and thus will result in a reduction in heating efficiency of heating wire. As a result, the responsibility as well as the sensitivity of the sensor will be degraded. To obtain a high sensitivity, it is desirable that the heating wire have a high heating efficiency. However, the reduction in the thickness of the heating wire results in an increase in current density, which results in an increase in the probability of failure due to electro-migration or the like.
In the case where the heating wire is made of a thermistor material as disclosed in Japanese Patent Laid-Open No. 61-235725 (1986), the thickness of the heating wire should be greater than 2 .mu.m to reduce the resistance to about 1 k.OMEGA.. However, this causes the thermal capacity of the heating wire to become too large and thus it is impossible to obtain good responsibility. Furthermore, when the sensor is mass-produced using a thermistor material, great variations occur in physical properties of the material, which causes a great reduction in production yield.